Most plug and receptacle combinations for power applications incorporate male pins as part of either the plug or the receptacle, and these are inserted into female sleeves in the other half of the plug and receptacle combination. However, to avoid any misunderstanding as to what is meant by the term "power" in this context, it relates to those applications where the connector is emplaced between the power source and the load so as to carry the bulk of the electrical current transmitted to the ultimate load, even if only a few amperes. Good electrical contact between pin and sleeve is usually achieved either by longitudinally splitting the pin and spreading those split portions slightly so that they compressively engage the female terminal when inserted therein, or alternatively, by longitudinally splitting the female and compressing the split portions slithly to achieve the same end objective. Several examples of these kinds of devices can be seen, for example, in U.S. Pat. No. 3,235,682 to Papworth and also in U.S. Pat. No. 3,585,323 to Appleton et al. As will be understood, the compressive contact between male and female terminals unfortunately causes a frictional force on the plug terminals tending to resist efforts to move the plug either into or out of mating engagement with the receptacle. And although this frictional resistance is reasonably small for very small plugs and receptacles, it increases to significant proportions as the electrical load transmitting capacity of the device increases. Conventional plug and receptacle devices capable of carrying as much as several hundred amperes or more become so bulky and difficult to manage that they may require several persons to insert the plug into the receptacle both because of the bulk of the plug (with its attached conductors) as well as because of the above mentioned frictional forces between male and female terminals that resist insertion. And, to avoid an even greater insertion force, the terminals are usually loosely mounted so that they center themselves upon insertion.
Beyond the traditionally popular pin and sleeve style of plug and receptacle terminals discussed above, another known but lesser used terminal design for plugs and receptacles incorporates butt contacts such as are shown, for example, in U.S. Pat. No. 3,982,804 to Marechal. In this construction the male terminals of the plug include butt contacts at their distal ends that engage receptacle butt contacts after substantial insertion of the plug into clearance holes provided therefor in the receptacle. However, although the Marechal design could have achieved a very low or perhaps even zero plug insertion force, at least over the initial and largest part of the insertion, that potential was never realized because the Marechal design utilizes the full plug insertion from start to finish to compress a spring within the receptacle. Compression of that spring is for the purpose of storing energy therein which is later utilized to facilitate a "quick break" of the male and female terminals to minimize the eroding effect of the arcs which are inevitably formed as electrical contact between the power transmitting butt contacts is broken. And although the Marechal device is technically a plug and receptacle because electrical contact is made directly between the plug and receptacle terminals by manually pushing the plug into the receptacle, the breaking of electrical contact is achieved by pushing a button which acts to release the compressed spring and cause the energy stored therein to drive the plug outwardly. Therefore, the physical act of breaking the circuit has considerable similarity with that of operating a pushbutton switch.
Other known electrical connectors also have a "quick-break" internal switching capability. For example, one of these other connectors is shown in U.S. Pat. No. 3,843,853 to Panek et al. Therein the pulling of an externally exposed pin or the melting of that pin, as caused by a buildup of heat therearound, automatically effects a disconnect of the internally connected, power transmitting terminals.
Where the power transmitted is relatively low, the mere physical act of pushing the plug into the receptacle is perhaps an acceptable technique in directly connecting the load with the power source. But, as the power rating of the plug and receptacle combination increases, it becomes increasingly desirable to provide some type of controlled switching arrangement within the body of the receptacle to both make and break contact consistently at optimum speed to thereby minimize arcing between the electrical contacts. Otherwise, for example, "diddling" of the contacts upon insertion will cause excessive damage thereto. Several ways of achieving a quick-make and quick-break are shown and described in various prior art. But because these devices usually position an internal switch in series with the plug and receptacle terminal connection, they tend to become excessively bulky, cumbersome, complex, and therefore, expensive.
For a variety of reasons it is also desirable to include polarizing means on plug and receptacle so that the plug will only insert into the receptacle at a given and predetermined axial orientation relative thereto. This ensures that the same male terminals always contact the same female terminals, which may be important, for example, depending upon whether the connector is wired for single or three phase current. Additionally, in the Marechal device described above, it is important that the plug housing always be at the same orientation relative to the receptacle housing when the power is connected so that external structure on the plug and receptacle housings will align and lock plug to receptacle against the full separation force of the compressed spring which would otherwise disengage them. Many power connectors utilize a conventional bayonet type of polarizing arrangement where by insertion of the plug is achieved by several distinct movements before the electrical connection is made. Initial insertion can only occur at a given angular orientation, but stops are encountered before electrical connection is made. However this initial insertion positions the plug for rotation (perhaps 45.degree.) to an intermediary and semicaptivated position defined by other stops. In some plug and receptacle designs this completes insertion whereas in others there may be yet another and further insertion step (as in Marechal) to make electrical contact. In any event, rotation of the plug in many designs also rotates the female terminals, and because they are usually directed connected to the power cables, those cables by flexing necessarily resist this rotation.
Because several or more identical plugs and receptacles might be in use proximate one another, but set up for example to transmit different voltages, it is desireable to include other polarizing means within the plug and receptacle to key a given plug only to a given receptacle. This will prevent the insertion of that plug into the wrong receptacle which might thereby injure the electrical equipment to which the plug is attached.
Referring once again to the Marechal device, that design includes a form of modifiable polarizing means that can be preset to ensure that a given plug will only mate with a given receptacle. Before use, the plug is disassembled and the plug terminals are revolved some fraction of one revolution relative to the plug housing, and then they are fixed in that new position to the plug housing. By compatibly rotating the receptacle terminals and fixing them relative to the receptacle housing, only that plug and receptacle will mate with one another unless identical changes are made to other plugs and receptacles.
When used in hazardous locations, other design restraints are normally imposed on electrical connectors. By "hazardous locations" it is meant that the connector device is used in an environment of potentially explosive gas or the like such that an exposed arc created by making or breaking electrical contact could ignite the surrounding atmosphere and thereby jeopardize life or property or both. Any arc that could occur under these conditions is thus preferrably confined within an internal space inside the connector that is defined by a specially designed housing. That internal space is not necessarily sealed from the external environment, and it thus may "breathe in" the same explosive fumes as exist externally of the connector, however any internal explosion caused by an internal arc is quenched by the connector housing before the flame front can propogate outside of the housing to the outside atmosphere and thereby cause a much larger and more dangerous explosion. The added versatility of being able to use a plug and receptacle in hazardous locations as well as in ordinary locations is an obvious advantage.
Connector devices used in hazardous locations are not only frequently of pin and sleeve design, but may also be classified as being of "dead front" construction. In fact, at least one of the prior art patents mentioned earlier is of dead front design. This simply means that insertion of the plug into the receptacle engages the pins of the plug with the sleeves of the receptacle which are not at that time connected to the power source. Hence they are deemed electrically "dead". Therefore, the insertion of plug pins into electrically dead receptacle sleeves will not create an arc. Then, after this initial connection, the receptacle sleeves are electrically connected to the power source within an explosion proof chamber by any of a number of known ways. This arrangement is normally quite safe in a hazardous location, although it does tend to add size and complexity to the resulting device because, once again, significant space is consumed in the multiplicity of spaces needed for the dead front connection, for the series-connected internal power connection and for whatever supporting paraphernalia or features are also included.
Other mechanism also frequently designed into power plugs and receptacles having switches are various forms of interlocks that act either to lock the plug to the receptacle when the power is on, that act to permit the circuit to be opened or closed only when the plug is fully inserted, and so forth. These interlock arrangements normally consist of mechanism auxiliary to the switch itself, and usually they are exposed to the atmosphere so as to be physically and chemically vulnerable.